Santorum won Louisiana, Mississippi, and Alabama; Gingrich won Georgia and South Carolina. Romney won a whole bunch of states where his victory is irrelevant, because those states will vote Democratic in the general.
The South -- the Republican heartland -- has no love for Romney. Huge numbers of voters there will vote for Romney because he has an (R) behind his name, but they won't knock on doors for him; they won't man the phones for him; they won't donate money to him. And on election day, they may not even work up enough enthusiasm to go to the polling station and vote for him.